1. Technical Field
The present invention relates to colloidal silicalite and zeolite crystals and, more particularly, to methods of removing entrained organic template molecules from within the crystals, and to applications of the detemplated crystals to produce thin films or membranes of zeolites or silicalites on substrates. The thin films may be used as separation membranes, catalytic membranes or low dielectric constant insulators in microelectronic devices.
2. Description of Related Art
One recent innovation in the formation of porous membranes has been the development of colloidal zeolites and silicalites. Sub-micron sized particles suspended in a liquid matrix are easily converted to a thin film on a substrate by a variety of methods. While membranes formed in this manner may ultimately be very useful in chemical catalysis and purification, the descriptions and examples in this application will emphasize films used in integrated circuit manufacture.
Increasing the speed and performance of integrated circuits (xe2x80x9cICsxe2x80x9d) typically calls for increasing the density of electronic components on the surface of a semiconductor wafer and increasing the speed at which the IC performs its functions. Increasing component density brings the charge-carrying circuit elements closer together, thereby increasing the capacitive coupling (crosstalk) between such circuit elements and delay in the propagation of signals through the conductors. Higher capacitance is detrimental to circuit performance, especially for high-frequency operation, as is typically encountered in telecommunication applications and elsewhere. One way of reducing the capacitive coupling between proximate circuit elements is to reduce the dielectric constant (xe2x80x9ckxe2x80x9d) of the insulator or insulating material(s) separating the coupled circuit elements.
It has been conventional in the fabrication of ICs to use dense materials as dielectrics, including silicon dioxide, silicon nitride and cured silsesquioxanes among others. The dielectric constant (k) of these materials typically lies in the range of approximately 3.0 to 7.0.
It is anticipated that the performance of future ICs is likely to be limited by resistive-capacitive (xe2x80x9cRCxe2x80x9d) delay occurring in the metallic interconnects of the IC, indicating that lower k dielectrics will be required for future ICs. As yet, the only fully dense materials with k less than about 2.4 are fluorinated polymers or fully aliphatic hydrocarbon polymers. However, such materials have not been shown to have sufficient thermal and mechanical stability to survive the thermal and mechanical stresses occurring during IC fabrication. In addition, these polymers typically have chemical properties that are similar in some respects to the chemical properties of photoresist materials commonly used in IC fabrication. Thus, chemical removal of photoresist layers without damaging dielectric layers becomes more difficult.
Several potential low k materials for IC dielectrics are materials that have a high degree of porosity. The open structure of such porous materials includes a significant amount of airspace. Therefore, the overall effective dielectric constant of the material lies between those of air and the fully dense material, typically significantly lower than that of the pure, solid material. Several general classes of porous materials have been described, including porous silicon dioxides.
Previous work by one of the present inventors relates to the use of colloidal silicalite crystals (xe2x80x9cCSCsxe2x80x9d) in forming spin-on dielectric coatings (interlayer dielectrics or xe2x80x9cILDsxe2x80x9d) in the fabrication of ICs, as described in U.S. application Ser. No. 09/514,966, filed Feb. 29, 2000, incorporated herein by reference. Silicalites are porous crystalline forms of silica having the same crystal structure as zeolites, as described, for example, by Edith Flanigen and Robert Lyle Patton in U.S. Pat. Ser. No. 4,073,865. Colloidal suspensions of silicalite crystals are described, for example, by Jan-Erik Otterstedt and Dale A. Brandreth, Small Particles Technology (Plenum Press, 1998), especially Chapter 5. See also The Synthesis of Discrete Colloidal Crystals of TPA-Silicalite-1 by A. E. Persson et. al. appearing in Zeolites, September/October 1994, pp. 557-567. See also Li, Q., Creaser, D. and Sterte, J., xe2x80x9cThe Synthesis of Small Colloidal Crystals of TPA-silicalite-1 with Short Synthesis Times and High Yieldsxe2x80x9d. in Porous Materials in Environmentally Friendly Processes, Ed. I. Kiricsi, G. Pxc3xa1l-Borbxc3xa9ly, J. B. Nagy, H. G. Karge, Stud Surf Sci. Catal., 125, 133 (1999), available online as a Master""s thesis at http://www.km.luth.se/kmt/theses/qlilic.pdf. CSCs offer the possibility of a porous, low k dielectric material that can easily be deposited on semiconductor wafers with standard wafer processing techniques and that can withstand subsequent etching, polishing and metallization steps.
However, CSCs are not suitable for film or membrane formation by themselves. A suitable binding agent must be used in cooperation with the CSC. That is, a CSC is typically deposited on the surface of a substrate along with a binding agent. Favored binding agents typically contain silicon and oxygen and crosslink at elevated temperatures, binding the CSCs into a porous ILD having adequate mechanical strength to withstand further processing. xe2x80x9cMonolithic filmsxe2x80x9d denote the films created by colloidal crystals having been bound together by a binding agent. For integrated circuits, binding agents based on silicon dioxide are desirable because of their proven compatibility with current IC processing steps, such as dielectric reactive ion etching and photoresist removal.
Silicalite crystals of an appropriate size for forming low dielectric constant films for integrated circuits are typically formed by stirring together a silica source, such as TEOS, and a so-called xe2x80x9cstructure directing agent,xe2x80x9d or SDA, in water. For many colloidal zeolites and silicalites, the SDA is a quaternary ammonia base. (See, for example, Tsapatsis and Gavalis, MRS Bulletin, March 1999, p. 32.) The mixture is stirred at sufficient temperature and for sufficient time for crystals of the desired size to grow. The choice of SDA is the strongest determinant of the crystal structure obtained. For example, tetrapropylammonium hydroxide (a quaternary ammonia base) yields the MFI structure, while tetrabutylammonium hydroxide (another quaternary ammonia base) yields the MEL structure. (Structure nomenclature as used in this application follows International Zeolite Association guidelines.)
To a lesser extent, crystal growth temperature determines the crystal structure obtained. For example, there are some quaternary ammonia species which yield a range of different structures, the predominant structure in a given batch being determined by the growth temperature.
As the crystals form around the SDA""s, the large SDA molecules eventually become entrained within the porous crystals. In order for the silicalite crystals to form an advantageous low dielectric constant film, the entrained molecules must be removed. This is because the SDA molecules themselves raise the dielectric constant, and also because they are often polar molecules. Polar molecules tend to attract water, which further increases the dielectric constant. Also, residual basic molecules in the film have the potential to cause unwanted reactions in deep-UV photoresist during subsequent processing. Similarly, to form a useful catalytic membrane or molecular sieve, the channels within the crystals must be cleared of obstructing molecules. The molecules have significantly larger diameter than any single channel in the crystal, in most cases, and so they cannot simply diffuse out. Thus, a need has been identified for a method to break the quaternary ammonia molecules into small, volatile byproducts that can diffuse out of the crystals.
The removal of entrained SDA molecules from zeolite or silicalite crystals may be referred to here as xe2x80x98detemplatingxe2x80x99. Because most industrial uses of zeolites and silicalites do not have severe thermal restrictions, a process referred to as xe2x80x9ccalcinationxe2x80x9d has evolved and been described. Calcination consists simply of exposing the crystals to high temperatures (typically 600-800xc2x0 C.) in the presence of air for up to 24 hours. U.S. Pat. Ser. No. 4,073,865, for example, refers to the calcination of crystals to decompose and burn the organic template. U.S. Pat. Ser. No. 6,177,373 teaches calcination as a means of detemplating thin zeolite films.
However, calcination is not applicable to many emerging applications, including the manufacture of integrated circuits. Integrated circuits contain either aluminum or copper wiring, both of which may deform and/or oxidize severely at temperatures above about 450xc2x0 C., providing a severe barrier to processing at elevated temperatures. Likewise, crystals suspended in a liquid medium, i.e., a colloidal suspension, will become fused together and unsuited for further processing if the liquid phase is allowed to boil away. Thus, a need for a lower temperature process has been identified.
In summary, for a process to be successful it must meet at least two requirements. First, the process must efficiently convert organic SDA""s to small byproducts that can easily diffuse out of the crystals. Second, it must not damage the crystal itself, or the device comprising the crystal.
In addition to ILDs, other applications for monolithic films of silicalite or zeolite nanocrystals include filtration membranes, molecular sieve membranes and catalyzation membranes. See, for example, the work of Anthony Cheethan, Gerald Ferey and Thierry Loiseau in Angewandte Chemie International Edition, Vol. 38, pp. 3268-3292 (1999). All of these applications likewise require template removal in order to allow free passage of molecules through the channels of the crystal. Additionally, for some of these applications, a lower-temperature detemplating process would be desirable.
The present invention relates to the formation of colloidal suspensions of silicalite and zeolite crystals (xe2x80x9cCSCsxe2x80x9d) which, among other applications, can be used in spin-coating of thin films for dielectric layers in integrated circuit (xe2x80x9cICxe2x80x9d) fabrication. CSCs are typically grown in an alkaline, aqueous medium, and deposited onto the IC, typically by spin-on deposition. However, CSC""s as grown contain entrained quaternary ammonia crystals or other structure-directing agents that must be removed to realize many of the advantages the CSCs provide. Thus, the objective of the present invention is to provide methods for removing quaternary ammonias or other structure-directing agents from the CSCs.
This patent describes several ways in which structure-directing agents such as quaternary ammonia molecules are removed by subjecting them to oxidative attack. In general, alkane groups, such as those comprising the ligands of the quaternary ammonia, may be oxidized to form carbon dioxide and water. The decomposition of tetrapropylammonium hydroxide (TPAOH) in the presence of elemental oxygen, for example, can proceed as follows:
(CH3CH2CH2)4N+OHxe2x88x92+370xe2x86x9212CO2+13H2O+NH3
CO2, H2O and NH3 can all diffuse easily through most zeolite or silicalite crystals. Thus, oxidative attack provides a means of removing the quaternary ammonia molecules from within the silicalite crystals.
This invention describes four methods for achieving detemplating which are compatible with IC manufacturing. Three methods perform the detemplating while the material is still in the colloidal suspension state, while the fourth is performed after the crystals have been bound together to form a film or membrane.
The first method is to use a combination of ammonia, water and hydrogen peroxide at elevated temperature after crystal growth and purification. A second method uses choline, hydrogen peroxide, water and a surfactant. A third method employs ozonated water, which may be used at the same point in the manufacturing process to achieve the same result.
Finally, the colloidal silicalite plus a binder may be applied to a substrate, such as a wafer on which integrated circuits are being formed, and the binder crosslinked to form a mechanically stable thin film. The wafer can then be placed in a chamber designed to create an oxygen-containing plasma above the wafer surface. Some important features of such a chamber are depicted in FIG. 1. Some of the reactive ions in the oxygen-containing plasma strike the porous silicalite-containing film, where they decompose the quaternary ammonia molecules and pump the decomposition products away. It is important to note that this process may also be used to simultaneously initiate crosslirking in the binder, causing it to bind the particles together. Thus, this latter method may simultaneously cure the film while it is detemplating the crystals.
There may be economic benefits to using the approaches described in this application, as compared with calcining, even if the structure in question is able to withstand high temperatures.